Skin is a complex composite of several layered tissues that function cooperatively. The skin is the largest organ of the human body and forms a barrier between the physical body and the outside environment. This barrier remains open and permeable to the environment, which allows for an exchange of heat, air, and fluids. The skin consists of three layers, namely: the epidermis, the dermis, and the subcutaneous layer.
The epidermis is the outer layer that covers the whole outside of the body. The epidermis contains numerous nerve endings that make the skin into one large sense organ that is operable to detect heat, cold, light, and touch. The outer layer of the epidermis, called the stratum corneum, is constantly being shed and, subsequently, replaced with new layers from below the epidermis. In the context of skin treatments, the stratum corneum operates as the rate-limiting barrier to percutaneous absorption and, further, fulfills one of its objectives by serving as a protective barrier. The stratum corneum is composed of dense layers of dead flattened cells. The stratum corneum is filled with fibrous protein keratin that is derived from the epidermis beneath. In its absence, for example when the skin is partially removed by some disease process, the absorption of drugs across the skin is increased.
The three layers of skin cooperate in a precise manner in order to produce an individual's outward appearance. Virtually all human beings are fixated on their appearance, albeit at varying degrees of interest. As can be appreciated, a person's age or health can be readily inferred from their skin. In other cases, the skin can signify previous acne issues, scarring (of any type), or other dermatological abnormalities that compromise a person's physical attractiveness.
Currently, there are thousands of treatment schemes, practices, and protocols that, ironically enough, are all claimed by their supporters to reverse the aging process and/or produce unblemished skin. Unfortunately, only minor fractions of the active substances in virtually all creams, gels, or lotions can penetrate the surface of the skin. The majority of the marketed products are simply wiped or washed off the face. Hence, the active ingredients of most cosmetic creams, how ever potent and wonderful, never really penetrate deeply enough into the skin to be effective. Note that the poor penetration characteristic of a given face cream is a natural and an expected physiological reaction of the skin.
Today, it is uniformly accepted that in order to achieve acceptable transdermal concentration of substances, the stratum corneum barrier must be breached or traversed. In order to increase skin permeability, a number of different approaches have been developed: ranging from chemical/lipid enhancers to electric fields employing iontophoresis and electroporation to pressure waves generated by ultrasound or photoacoustic effects. With the exception of chemical/lipid enhancers, all of these methods share the common goal of attempting to disrupt the stratum corneum structure in order to create holes big enough for molecules to pass through the stratum corneum.
The human skin represents an attractive alternative method for the delivery of drug administration because it can provide a patient-friendly interface for systemic drug administration, an avoidance of first-pass metabolism, and a sustained and controlled delivery of a substance. Previously, the only means of delivering molecules through the skin was by either hypodermic needles or transdermal patches.
Hypodermic injection techniques are demanding for the doctor and problematic for the patient, as they cause local pain and trauma. Hypodermic needles effectively deliver any drug at virtually any rate across the skin, but are limited by pain, the need for medical expertise (which increases cost), and the difficulty of having controlled delivery over long periods of time.
In contrast, transdermal patches largely eliminate many of these limitations, but suffer from an inability to deliver most drugs across skin at useful rates. Until today, the only drugs that have been marketed in a patch-delivery method are scopolamine, nitroglycerine, nicotine, clonidine, fentanyl, estradiol, testosterone, lidocaine, and oxybutinin. The reason why these drugs have been successfully delivered in a patch form is that their small molecular size (e.g. smaller than 500 Daltons) allows for a limited passive diffusion through the stratum corneum.
In summation, the disadvantages of existing skin treatment systems and devices in the market place are noticeable. The problem with simply applying any cosmetic material on the skin is that the stratum corneum is impregnable or impermeable. Elements simply do not go through this barrier, which is why most current cosmetic products are inactive, as they only linger on the surface of the skin and fail to reach the point where they could actually be beneficial. Hence, the challenge is not only to find an optimal material to be delivered to the certain layers of the skin (e.g. the dermis), but how to get the chemical agent to the point where it is most effective. This represents a classic pharmacological problem. Concisely stated, the objective is to find the right agent (having the right concentration) and to employ the right delivery system in order to reach the right target cell location. Details concerning each of these objectives are discussed more fully below in the context of the present invention.